There is an increased demand for elastomeric compositions having improved heat and fluid resistance in automotive and industrial applications. In automotive applications, the new sophisticated design and engineering of engine compartments and under-the-hood parts requires the use of elastomers that can withstand higher service temperatures and long term aging. Similarly, industrial applications are following a similar trend where elastomers are expected to have high performance and function in harsh conditions.
The heat resistance of an elastomer is defined as the maximum temperature at which a given elastomer is capable of operating for an extended period of time, while still retaining its properties. The physical and chemical properties of elastomers exposed to heat and/or air can change and that change can include: (a) additional crosslinking resulting in higher crosslink density and embrittlement of elastomers, (b) chain scission leading to a reduction in chain length and average molecular weight, leading to softening of the material; and (c) chemical alternation of the polymer chain by the formation of polar or other groups.
At elevated temperatures, molecules absorb heat energy leading to covalent bond cleavage and material degradation. Unsaturation in the polymer chains will make the materials more susceptible to heat because the energy required to break the second bond of C═C to form active radicals is relatively low. This behavior is clearly evident by the low thermal degradation temperatures of butyl rubber, nitrile rubbers and styrene-butadiene rubbers.
When nitrile (NBR) is hydrogenated to HNBR using a metal catalyst at designated temperatures and pressures, a new polymer is formed that has excellent heat and oil resistance and its properties can be controlled by varying the acrylonitrile level, residual double bonds, and molecular weight. Hydrogenated nitrile elastomers have excellent chemical, oil, and solvent resistance, as well as good aging and heat resistance. HNBR is an improvement over the limitations of NBR, in terms of degradation when exposed to high underhood temperatures in automotive fuel-line components. It also has reduced problems associated with cracking and reduced physical properties. Increasing hydrogenation levels give higher heat and ozone resistance. HNBR is widely used in the automotive market. Examples are belts, hoses, static and dynamic seals. Other areas of application include seals for oil field exploration and processing, and rolls for steel and paper mills.
In addition to the basic polymer chain structure, the vulcanization system also plays a role in the thermal stability. Elastomers vulcanized with peroxides, which form C—C crosslinks, have higher heat resistance as compared to sulfur cured elastomers. This is because the bond energy of C—C is much higher than C—S and polysulfide bonds. Also, heat resistance of sulfur cured elastomers is affected by the length of the sulfur crosslinks. Because of the much weaker polysulfide bonds, vulcanizates with long sulfur crosslinks are more vulnerable to deterioration under heat and oxygen. However, peroxide crosslinking requires special attention to the selection of compounding ingredients. Materials such as plasticizers, oils, and acidic materials can decrease the crosslinking efficiency by competing with the polymer for the free radicals produced by peroxides.
There are many methods of improving the heat resistance of HNBR compounds using antioxidants that have minimal interference with peroxide curing, high pH fillers and additives, as well as acid acceptors. Other methods include the use of synergistic stabilizer systems composed of metal salt of a secondary amine as disclosed in U.S. Pat. Nos. 6,947,526; 6,451,902; 7,005,467; U.S. Patent Publ. No. 2004/092634; U.S. Patent Publ. No. 2005/0143522.
The present invention provides a novel elastomeric composition containing an acid acceptor, and a silane-modified mineral additive. Compositions according to the present invention have superior heat resistance at 150° C. and higher temperatures, compression set resistance, and improved processability as evidenced by a lower compound Mooney, shows excellent heat and compression set resistance after long term aging. The inventive composition also has improved processability.